1. Field of the Invention
The present invention relates to an illumination device and a display device including the illumination device, and, in particular, to an illumination device provided on a backside of a display panel and used as a backlight of a display device, and to a display device including such an illumination device.
2. Description of the Related Art
It has been known that a liquid crystal display device is widely used for office automation equipment, an in-car TV, a monitor of a video camera and the like, because the liquid crystal display device has a lightweight and thin body, and has low power consumption. Unlike a display device which uses an element that emits light by itself, such as a CRT, a PDP (plasma display panel), and an EL (electroluminescence), the liquid crystal display device uses a liquid crystal display element that does not emit light by itself. Therefore, a transmissive liquid crystal display device has a planar illumination device called a “backlight” on a backside of the liquid crystal display element. At each pixel, the liquid crystal display element controls transmission of light from the backlight, so as to display images.
The backlight is roughly classified into a “direct” backlight in which a plurality of rod-like light sources, such as fluorescent tubes, are provided directly below the liquid crystal display element, and an “edge light type” backlight in which a light source is provided at an end of a light guide plate, and the light guide plate leads light from the light source to the liquid crystal display element.
A general arrangement of the direct backlight is schematically illustrated in FIG. 16. In FIG. 16, a backlight 40 is provided on a backside of a transmissive liquid crystal display panel 48, the backlight 40 including a plurality of fluorescent tubes 41 positioned at prescribed intervals, a case 43 for containing the fluorescent tubes 41, and an optical sheet 45 (such as a diffuser sheet or a prism sheet) provided between the fluorescent tubes 41 and the liquid crystal display panel 48.
With this arrangement, the direct backlight 40 can adjust its luminance easily by increasing or decreasing the number of the fluorescent tubes 41, which act as a light source, so that the direct backlight 40 can achieve a high luminance easier than the edge light type backlight. Consequently, the direct backlight 40 is usually used for a large liquid crystal display device.
However, with the direct backlight 40, the light sources (fluorescent tubes) 41 are provided directly below the liquid crystal display panel 48 at discrete intervals. Therefore, regions directly above the light sources 41 have a high luminance, while regions corresponding to the intervals between the light sources 41 has a low luminance, thereby resulting in an occurrence of luminance unevenness.
The longer a distance A between the light sources 41 and the liquid crystal display panel 48 is, and the shorter a distance B between the light sources 41 is, the less the luminance unevenness there is. In short, the smaller a ratio R of the distance B to A (=B/A) is, the less the luminance unevenness there is, and the bigger the ratio R is, the more the luminance unevenness there is. Therefore, the luminance unevenness can be reduced with a longer distance A between the light sources 41 and the liquid crystal display panel 48 or with a shorter distance B between the light sources 41 by increasing the number of the light sources 41.
However, such a longer distance A between the light sources 41 and the liquid crystal display panel 48 requires the backlight 40 or a display device to be thick, thereby resulting in poor commercial attractiveness. Moreover, shortening the distance B between the light sources 41 by increasing the number of the light sources 41 results in an increase in cost, again thereby resulting in poor commercial attractiveness as well.
In order to reduce the luminance unevenness, Japanese Unexamined Patent Application Publication No. 2002-122863 discloses a backlight 50. The backlight 50 includes, as illustrated in FIG. 17, a protruding portion 52 formed between light sources 51. The protruding portion 52 has a triangle cross sectional shape and a light reflectivity. Thereby, the luminance unevenness is reduced in the backlight 50.
Further, Japanese Unexamined Patent Application Publication No. 2000-310776 discloses a backlight 60. The backlight 60, as illustrated in FIG. 18, includes an auxiliary light source 64 including a primary light source 62 and a light guide plate 63, the auxiliary light source 64 provided to face a liquid crystal panel across light sources 61. Thereby, the luminance unevenness is reduced in the backlight 60.
Furthermore, according to Japanese Unexamined Patent Application Publication No. 2000-310776, on a surface of the light guide plate 63, scattering dot patterns are formed to take out light outwardly, the light entering the light guide plate 63 from the primary source 62. The scattering dot patterns are sparsely arranged in a region directly below the light source 61, and densely in regions corresponding to the intervals between the light sources 61. As a result, it is possible to have a low luminance of the light coming from the light guide plate 63 in the regions directly below the light sources 61, and have a high luminance of the light in the regions corresponding to the intervals between the light sources 61, so as to reduce the luminance unevenness.
However, with the backlights disclosed in Japanese Unexamined Patent Application Publication No. 2002-122863 and Japanese Unexamined Patent Application Publication No. 2000-310776, the luminance unevenness is not reduced sufficiently. Specifically, with the backlights disclosed in Japanese Unexamined Patent Application Publication No. 2002-122863 and Japanese Unexamined Patent Application Publication No. 2000-310776, in a front direction (a normal direction of the display surface of a display device), the luminance unevenness is reduced sufficiently, however, in oblique directions (oblique directions to a normal direction of the display surface), the luminance unevenness is not reduced sufficiently, thereby resulting in the luminance unevenness of the backlights, as a whole.